The purpose of this project is to study physiological and pathological aspects of the renin-angiotensin system, with emphasis on its role in circulatory homeostasis and development. During this period research has focused on the characterization and mechanism of action of novel AII receptors previously identified by this laboratory in the fetus. AII receptor subtypes were studied using receptor subtype-specific AII antagonists. A new finding was that AII receptors in tissues where binding remains through adulthood are of subtype-1 (AT1), which are coupled to calcium-phospholipid dependent signaling systems and are sensitive to guanyl nucleotides. In contrast, AII receptors transiently expressed in the skin and skeletal muscle are type-2 (AT2), with unidentified signaling mechanisms and binding insensitive to guanyl nucleotides. Studies during postnatal development showed a large proportion of AT2 in the adrenal and smooth muscle after birth, which decreased to adult levels after 2 weeks. The presence of uncoupled AT2 may contribute to the low sensitivity of aldosterone and pressor responses to AII in neonates. Studies in cultured fetal skin fibroblasts, showed converse changes in receptor subtype during culture, with marked decreases in AT2 and increases in AT1. Treatment of the culture with actinomycin D markedly increased AT2 content, suggesting posttranscriptional regulation of this receptor subtype. In contrast to other systems, AII increased cAMP accumulation in fetal fibroblasts by indirect activation of adenylate cyclase activity. This effect as well as the stimulation of inositol phosphate formation were mediated by AT1. Studies in the central nervous system were extended to the analysis of AII receptor subtypes during development. Receptors in areas related to control of blood pressure and water intake were AT1 at all ages, while most of the binding transiently expressed in motor and sensory areas during the first 2 weeks were AT2. Studies on the role of AII receptor subtypes on aldosterone secretion showed that adrenal glomerulosa cells contain predominantly AT1, but the content of both receptor subtypes increase during sodium restriction. In isolated adrenal glomerulosa cells from normal or sodium restricted rats, the AT1, but not the AT2, antagonist inhibited the effects of AII on aldosterone production, inositol phosphate formation and cAMP accumulation, indicating that AT1 are responsible for stimulation of steroidogenesis. The abundance of AT2 and its expression at critical times in development, suggest that this receptor subtype has a role in cellular growth and maturation.